1. Field of the Invention
The present invention relates to a rare earth bonded magnet comprising rare earth magnet powder bonded with a resin, a rare earth magnetic composition which is used for the production of the rare earth bonded magnet, and a method for manufacturing the rare earth bonded magnet using the rare earth magnetic composition.
2. Description of the Related Art
Generally, rare earth bonded magnets of various shapes are molded from compounds of rare earth magnet powders and binder resins or organic binders by pressure molding, such as compaction molding, injection molding, or extrusion molding.
In compaction molding, a bonded magnet is produced by packing a compound into a mold, compacting it to form a green body, and then heating it to harden the thermosetting binder resin. Since the compaction molding method enables molding with a smaller amount of binder resin as compared to other molding methods, the magnetic properties of the resulting magnet are enhanced. However, molding versatility regarding the shape of the magnet is restricted and production efficiency is low.
In injection molding, a bonded magnet is produced by melting a compound and then injecting the flowable melt into a mold of a specified shape. The injection molding method allows high molding versatility regarding the shape of the magnet, and magnets having irregular shapes can be easily produced. However, since injection molding requires a high level of melt fluidity, a large amount of binder resin must be added. Thus, the binder resin content in the magnet is increased which results in lowered magnetic properties.
In extrusion molding, a bonded magnet is produced by melting a compound into an extruder, extruding the compound and cooling the compound from a die of the extruder into a long molded body. The long body is then cut into specified lengths. The extrusion molding method has advantages of both compaction molding and injection molding. That is, in extrusion molding, the shape of the magnet can be varied to some extent by using an appropriate die, and a thin-walled or long magnet can be easily produced. Further, the resulting magnet shows enhanced magnetic properties due to the reduced amount of binder resin because high fluidity of the melt is not required, unlike the injection molding.
Thermosetting resins, such as epoxy resins, have been generally employed as binder resins in compounds, as disclosed in Japanese Patent Publication (Examined) Nos. 56-31841 and 56-44561. The content of the thermosetting resins is low from 0.5 to 4.0 percent by weight due to its high fluidity.
However, there are many points which are not clarified concerning optimum conditions for molding using thermoplastic resins as binder resins, e.g. the effect of the rare earth magnet powder content on the moldability, magnetic properties, and mechanical properties of the bonded magnet.